NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Sextant vs. Digital Camera
From: Greg Rudzinski
Date: 2008 Aug 8, 17:21 -0700
From: Greg Rudzinski
Date: 2008 Aug 8, 17:21 -0700
George, Very good observations of my post. I did a high altitude check using the Cassens and plath with a 4 power scope and had everything within a mile and a half with an extreme difference of half a minute. So yes there was refraction but that should be expected at very low altitude. I did not adjust for flatness so there is another small factor. I do have some pictures and I will let you be the judge. I believe the pixels are to the nearest pixel which implies a half pixel potential error. Keep in mind that all of this is done with a $125 camera. On Aug 8, 4:09�pm, "George Huxtable"wrote: > Thanks to Greg Rudzinski for posting his interleaved sextant and camera > observations of a setting sun; just the thing we need to assess the accuracy > of one against the other. > > His stated conclusion was this- > > "CONCLUSION > > The results demonstrate the ability of a 7 mega pixel camera at optical zoom > 4 power �to accurately function �as a sextant " > > However, I think we need to examine the details first, before agreeing with > that conclusion. > > Because this also is stated- > > "Sextant observations �observed with CASSENS & PLATH using sight tube and > whole horizon mirror." > > And when we compare the results of the camera and a sextant USED WITH A > SIGHT TUBE (i.e., without a telescope) we see that the camera gives an > average difference between calculated and observed altitudes of 3.5', > against 2.1' for that sextant, �the scatter being about the same, in the two > cases,. of 2.2' or 2.1' between extreme values. Not a wonderful performance, > for an observation made from firm ground in what are described as ideal > conditions. Neither were the sextant observations anything to be proud of; > but that may be due to the difficuly of working without a telescope, > combined with the problems inherent in correcting for refraction at such low > altitudes, and perhaps an unusual value (unknown) for local dip. > > In which case I would rephrase that conclusion to state only that the camera > gave a mediocre performance which was significantly worse than that of a > sextant without a telescope. > > I wonder if Greg Rudzinski has retained any of the images from that set; in > particular, the image(s) which were used to obtain the Sun diameter, and > from that the overall calibration factor. I am interested to see how sharp > and well-defined the Sun limbs were, and whether there was any leeway in > assessing the Sun's diameter, at 108 pixels, by choosing a different > brightness contour for the fit to the disc. After all, changing that contour > by just one pixel, from 108 to 107 pixels, would cause a difference of > nearly 1% to the calibration factor, and shift the first of the camera > observations in the set by more than 3 arc-minutes > > Finally, I wonder if Greg has made any assessment for non-linearity of the > angular scale that was used, and made any correction based on that > assessment. > > George. > > contact George Huxtable at geo...@huxtable.u-net.com > or at +44 1865 820222 (from UK, 01865 820222) > or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---